(1) Field of the Invention
This invention relates to a fill material for an underground void; and to a method of backfilling an underground void.
(2) Description of Prior Art
In mining, backfill is often necessary as part of a particular mining method for ground stability or as a platform to work from. In general, the milling of ore produces a waste stream of finely ground rock particles, ranging in size from 425 microns to 10 microns and finer, often referred to as tailings. These tailings may be employed as the principal component of backfill.
Natural sand rock material ranging from less than 15 cm to 0.23 cm and some other type of aggregate as well as combinations of any of these materials, referred to as aggregates may also be employed as the principal component of backfill.
Alluvial sand is also employed as backfill, especially in cases where mine tailings are not readily available.
Thus, backfill generally comprises mill or mine tailings, a waste product from mine beneficiation, natural sand, rock material or some other type of aggregate or some combination of any of these materials, referred to as aggregates.
Not all backfill must be free standing, but when consolidation is required, the aggregates are mixed with a slurry comprising a binder and water. When consolidation is required, the aggregates are mixed with this slurry of binder and water. Common binders can consist of a combination of one or more of the following, a variety of Portland cement, iron blast slag, fly ash and lime. On some occasions flocculants, water reducers, accelerators, gelling agents, such as sodium silicate, or other specialty products are added to the backfill to: enhance flow, retard set-up of the binder, minimize the loss of fine particles, accelerate early strength and absorb water.
For hydraulically placed material and paste backfill, boreholes and pipes are used to transport the backfill to its final destination. For rock backfill, generally trucks or conveyor belts are used to deliver the fill to its planed site of application.
In order for a binder to be effective in any backfill application, the high early strength (3-5 days) and or long term strength (28 days) must satisfy a strength criteria highly dependent on the widths and heights of walls left unsupported after mining of a particular section (stope) is complete. Other important criteria that affect backfill strength include: backfill aggregate size distribution, binder concentration levels and the water content of the backfill.
The mine tailings and other aggregates are essentially inert to reaction with the binder and the water, although sulphates, magnesium and other metal oxides and compounds found in mine tailings may occur in quantities that affect the effectiveness of the binder. In practice the binder sets in the presence of the water to form a solid mass which binds the aggregate of the backfill.
In practice the fill material which is comprised of aggregate, binder and water is formed into an aqueous mobile slurry or paste which is gravity fed or pumped through pipes or otherwise introduced into the void. In place in the void the binder and water react, solidifying into a mass which binds the backfill. This solidification process may extend over several weeks.
The solidified material provides support in the void (stope), for example, to maintain the integrity of adjacent tunnels employed to provide access to new mining areas. Typically the voids are underground sites from which metal ore has been mined and the mine tailings component of the backfill introduced into the void may be derived from the metal ore previously removed from the mining site which resulted in formation of the void.
The use of the backfill in this manner also provides a ready means of disposing of the waste mine tailings while maintaining the structural integrity of the mine substructure.
Portland cement is usually a significant component of binders commercially employed for backfill which includes mine tailings as described here. However, it would be advantageous to mining companies to utilize binders that outperform Portland cement, and that employ only low amounts of Portland cement or no Portland cement.
U.S. Pat. No. 6,402,833 describes a fill material for an underground void which employs a binder based on ferrous slag and cement kiln dust, and optionally lime kiln dust. This binder was found to outperform Portland cement as a binder for backfill, in many conditions.